Unemployment has not returned to Great Depression levels but at 9.1% of the workforce it is still at levels that will have nerves jangling in the White House. The last president to be re-elected with unemployment above 7.2% was Franklin Delano Roosevelt.

The US is a country with serious problems. Getting on for one in six depend on government food stamps to ensure they have enough to eat. The budget, which was in surplus little more than a decade ago, now has a deficit of Greek-style proportions. There is policy paralysis in Washington.

The assumption is that the problems can be easily solved because the US is the biggest economy on the planet, the only country with global military reach, the lucky possessor of the world's reserve currency, and a nation with a proud record of re-inventing itself once in every generation or so.

All this is true and more. US universities are superb, attracting the best brains from around the world. It is a country that pushes the frontiers of technology. So, it may be that the US is about to emerge stronger than ever from the long nightmare of the sub-prime mortgage crisis. The strong financial position of American companies could unleash a wave of new investment over the next couple of years.

Let me put an alternative hypothesis. America in 2011 is Rome in 200AD or Britain on the eve of the first world war: an empire at the zenith of its power but with cracks beginning to show.